Local Number Portability (LNP) allows telephone service subscribers to retain their same directory numbers when they change service locations and/or service providers. Telephone number portability was mandated by the Telecommunications Act of 1996.
Local number portability is made possible by a location routing number (LRN). When a subscriber's local service is moved to another service provider, such as at another switching office, the ported directory number of the subscriber is associated with the LRN of the ported-to switching office in a number portability database, which in the United States is administered by the Number Portability Administration Center (NPAC). The NPAC distributes number portability data to service provider via local service management systems (LSMSs). The LSMS of each service provider is used to provision the service provider's number portability database. When a call is made to a ported subscriber, the originating switching office has a software-implemented mechanism, referred to as a trigger, which detects that the called party directory number is ported and queries a number portability database. The number portability database returns the LRN of the recipient or ported-to switching office and the originating switching office routes the call to the ported-to switching office.
Because LNP is becoming more common, LNP trigger functionality, or, more generally, LRN based query and response functionality, is becoming a standard feature of switching office equipment. Because LNP trigger functionality is available in most switching office equipment, it may be desirable to use LNP triggers to route calls for reasons other than LNP. However, LNP triggers are typically used for LNP.
One situation for which it may be desirable to use LRN based originating switching office query and response functionality to route a call is when the called subscriber is an IP multimedia subsystem (IMS) subscriber. One possible mechanism for an originating switching office to recognize a call to an IMS subscriber is to provision the switching office with an IMS trigger so that the switching office would query an IMS database to obtain LRN routing information for the IMS subscriber. However, provisioning new switching office triggers can be costly, especially as the number of IMS subscribers increases.
Accordingly, there exists a need for using an LRN based query and response mechanism to route calls to IP multimedia subsystem (IMS) subscriber.